My basic rule of thumb is that anything you have ever seen sold makes $100,000+ for someone because otherwise it couldn't justify having multiple people devoted to it. Consider bingo cards, for example: Scholastic, a Real Company (TM) which has Real Capital Expenditures (TM) and pays Real Salaries (TM) publishes on-dead-tree bingo cards through stores which habitually do 100% markups over wholesale. So for Scholastic to expend their corporate efforts to author, print, and stock that, it must sell tens of thousands of units per year.

Bingo Card Creator is strictly superior to about half of their bingo products (can't do pictures), costs less at the quantities my teachers use, and has such an astoundingly better cost structure (what, you mean I don't leak 50% of the MSRP to the retailer?) that I'm essentially guaranteed to make money on sales.

After that, it is only a matter of marketing. (He says with a twinkle in his eye.)

Incidentally, I found a niche because someone came up to me and said "Oh bugger I would pay if this problem would go away." When is the last time you talked to someone who wasn't a techy but had money and problems?